With the emergence of 3rd generation (3rd Generation, 3G) mobile communications and more advanced digital mobile technologies, demands for a mini base station increase, and soon the picocell may be used at home as a femtocell. The femtocell has a high requirement that time synchronization is implemented in a low cost, and a mobile service requires that precision of time synchronization reaches a microsecond level.
During signal transmission in an X digital subscriber line (X Digital Subscriber Line, XDSL) system, uninterrupted symbols are transmitted one by one, and no obvious boundary exists between the symbols. Therefore, receiving end equipment is difficult to find a strict synchronization point to mark a moment when the receiving end equipment receives the signal sent by transmitting end equipment, and a moment when the transmitting end equipment receives the signal sent by the receiving end equipment.
In addition, uplink and downlink delays in an XDSL channel are different as a channel between central office (Central Office, CO) equipment and customer premises equipment (Customer Premises Equipment, CPE) is complicated, and the signal in the channel needs to pass through a physical media dependent (Physical Media Dependent, PMD) layer having an analog circuit and also having a digital signal processing circuit, a physical media specific-transmission convergence layer (Physical Media Specific-Transmission Convergence Layer, PMS-TC) with complicated Reed-Solomon (Reed-Solomon, RS) coding/decoding and interleaving/de-interleaving, and a transmission protocol specific-transmission convergence layer (Transmission Protocol Specific-Transmission Convergence Layer, TPS-TC) with complicated protocol processing. Even if time synchronization is performed in the PMD layer, an analog circuit at the central office, a cable, and an analog circuit at the remote end also need to be pass through, and a common digital circuit and a digital signal processing circuit between the central office and the remote end further need to be passed through, so that the downlink delay from the central office to the remote end is not necessarily equal to the uplink delay from the remote end to the central office. In the prior art, in the XDSL link, a difference between the downlink delay and the uplink delay may be much greater than 1 μs, and at this time, a time error after recovering in the prior art may be much greater than 1 μs. However, no synchronization method in the prior art can meet the high-precision requirement of the XDSL system on the clock synchronization.